The present invention relates to a communication apparatus employing a digital signal, and particularly to a .pi./4 shift QPSK modulator suited for reducing the circuit scale and for integrating the components into circuits and to a communication apparatus used therewith. Heretofore, a QPSK system has widely been used as a digital signal modulation system. In the QPSK system, there is the possibility that a phase vector passes through a zero point when the phase of the digital signal varies. A problem is therefore encountered with such a system in a radio communication system that there is a high possibility of deteriorating the S/N ratio. In order to solve such a problem, a .pi./4 QPSK modulation system has been proposed wherein an axis for shifting the phase of data is rotated for each symbol to avoid the passage through the zero point. This system is generally described in the article entitled "Highly Efficient Digital Mobile Communications with a Linear Modulation Method" by Akaiwa et al, IEEE Journal on Selected Areas in Communications, pp. 390-395, Vol. SAC-5, No. 5, June 1987, but the article does not provide a detailed description sufficient to construct the system.
FIG. 4 is a diagram showing the phase of data subjected to QPSK modulation. FIG. 20 is a block diagram depicting the structure of a conventional QPSK modulating circuit which includes a mapping circuit 40 for calculating components (hereinafter called "phase information") of I and Q which intersect each other at right angles from 2-bit digital signals, a filter circuit 41 for cutting off a high-frequency component of the phase information to subject the same to band-limitation and a normally-used cross or quadrature modulator circuit 16 coupled to an oscillating circuit 18. The inputted digital signals are disposed in four phase positions in the mapping circuit 40 as shown in FIG. 4. More specifically, the phase information (I, Q) corresponding to each phase is outputted from the mapping circuit 40. The high-frequency components of the phase information are cut off to be band-limited in the filter circuit 41. A filter having a roll-off characteristic is normally employed in the filter circuit for canceling out the interference between codes. Since the roll-off filter has a complicated characteristic, a digital filter is often used. It is necessary to execute an arithmetical operation on a signal at high speed because this digital filter performs the processing on the signal by multiplication and addition operations. For this purpose, difficulties are encountered with an apparatus to be operated by a battery such as a portable transceiver in view of the power to be consumed.
In order to overcome such a problem, a technique has been proposed that the filter circuit and additionally the quadrature modulator circuit are replaced with a ROM by storing the results of arithmetical operations in the ROM, in advance, and providing input data as addresses for the ROM as described in Japanese Patent Application Laid-Open No. 24763/1978. This technique makes the complicated structure of the processing circuit and the requirement for executing the data processing at high speed unnecessary. Although the aforementioned prior art omits complicated multiplication and addition circuits and therefore makes the performing of an arithmetical operation on a signal at high speed unnecessary, no consideration has been given to the processing corresponding to the phase shifting as the transition of a state of the phase of the .pi./4 shift QPSK signal as shown in FIG. 3. Further, the prior art is accompanied by the drawback that the capacity of the ROM is exponentially increased in accordance with the number of taps of the filter and the number of phase states thereof.